Light modulators are well known in the art. In general, light modulators modulate light entering the light modulator in accordance with the level of a control supplied to the light modulator. Depending on the construction of the light modulator, the level of the control will dictate (1) the extent of a change in the amplitude of the light emerging from the light modulator, or (2) the extent of a change in the polarization of the light emerging from the light modulator, or (3) the extent of a change in the phase of the light emerging from the light modulator, etc.
Light modulators can be constructed so as to pass or reflect the modulated light, and can use different surfaces of the light modulator to receive the control. For example, FIGS. 1-3 schematically illustrate three common light modulator constructions.
One type of light modulator of special interest in the present invention is a so-called optically addressed light modulator. In this type of light modulator, the control for the light modulator is optically based.
Individual light modulators can be grouped together in different ways so as to provide particular constructions. One construction of special interest in the present invention is a so-called spatial light modulator, which comprises a two dimensional array of individual light modulators.
The present invention is principally concerned with optically addressed spatial light modulators (hereinafter frequently referred to as "OASLM's").
There are numerous commercial applications for OASLM's.
For example, OASLM's can be used to boost a low light source, e.g., for night vision applications. In this situation, the light obtained from the real-life light source is used as the control. More particularly, light from a robust, regulated light source is fed into the light modulator, and the control is used to modulate the light from the regulated light source into a corresponding modulated output.
OASLM's can also be used to convert incoherent light into coherent light. This can be important in some optical systems which need to use coherent light. In this situation, the incoherent light is used as the control. More particularly, coherent light from a regulated light source is fed into the light modulator, and the control is used to modulate the coherent light from the regulated light source into a corresponding modulated output.
OASLM's can also be used to convert the wavelength of light. For example, the OASLM could read infrared light and convert it to visible light.
OASLM's can also be used as a short term storage device, e.g., real-time holographic images can be stored for several hundred microseconds in an OASLM.
In practice, in many applications it has been found desirable to construct OASLM's without having discrete pixel structures within the OASLM. In essence, it has been found desirable to construct OASLM's having a virtual pixel structure. See, for example, FIG. 4, which schematically illustrates an OASLM having a virtual pixel structure.
When constructing an OASLM, it is generally important that there be good lateral isolation between adjacent virtual pixels, so as to avoid cross-talk and achieve good device resolution. It is also generally important that the OASLM have good sensitivity, so that a low-level light source can be used as the control. The OASLM should also be relatively efficient, in the sense that the light modulator should absorb relatively little of the light which is to be modulated. And the OASLM should also be able to provide a good degree of modulation, so as to deliver an output which is more easily discernible. It is also generally important that the OASLM be able to operate at a relatively high readout-to-control ratio, so that the light modulator's output can be more easily used and so that the intensity-related requirements of a following optical or electronic subsystem can be relaxed. The OASLM should also be able to operate at a relatively high speed, so that the OASLM can be used in a broad range of device applications. And the OASLM should, ideally, have a monolithic construction, so that the OASLM can be more easily fabricated.
Looking next at FIG. 5, an OASLM 5 is shown. OASLM 5 generally comprises a pair of transparent electrodes 10, a pair of buffer layers 15 and an electrooptic region 20.
Transparent electrodes 10 typically comprise a transparent conductive material (e.g., cadmium tin oxide or indium tin oxide) or a semiconductor material (e.g., n-doped or p-doped gallium arsenide or gallium aluminum arsenide).
Buffer layers 15 typically comprise dielectric layers (e.g., phosphate silica glass).
Electrooptic region 20 typically comprises a semiconductor electrooptic material, e.g., multiple quantum wells (MQW's) made of alternating layers of gallium aluminum arsenide and gallium arsenide.
Transparent electrodes 10 apply a uniform field across electrooptic region 20 when OASLM 5 is connected to an appropriate voltage source 25. When control light thereafter enters OASLM 5 and strikes electrooptic region 20, the control light interacts with the electrooptic material, causing electron formation. This electron formation leads to a change in the electric field within the electrooptic region 20, which in turn leads to a change, on a localized basis, in the electrooptic material. This localized change in the electrooptic material in turn leads to a change, on a localized basis, in the modulation of the readout beam as the readout beam enters OASLM 5 and passes through electrooptic region 20.
In OASLM 5, buffer layers 15 serve as electron binders, essentially keeping the electrons from (1) being "sucked" out of the device longitudinally, which causes a loss of device efficiency, and (2) moving laterally, which causes each virtual pixel to be less well defined, and hence leads to a loss of device resolution.
In OASLM 5, electrooptic region 20 simultaneously acts as both (1) the detector of the control light, and (2) the modulator of the readout beam.
With OASLM 5, if electrooptic region 20 is made relatively thick, a relatively large amount of electrooptic material will be present in the device and the OASLM can provide good sensitivity and good modulation. However, since a relatively thick electrooptic region 20 results in the buffer layers 15 being spaced relatively far apart, substantial lateral electron migration can occur, which results in reduced device resolution.
On the other hand, if electrooptic region 20 is formed relatively thin, so that buffer layers 15 are spaced relatively close together, lateral electron migration will be restricted and the OASLM will provide good resolution. However, since the electrooptic region 20 is relatively thin, a relatively small amount of electrooptic material will be present in the device and the OASLM will have relatively poor sensitivity and relatively poor modulation.
In addition to the foregoing, with OASLM 5, since the same electrooptic region 20 serves as both the detector for the control light and the modulator for the readout beam, a relatively low readout-to-control ratio must be used so as to avoid having the readout beam "wash out" the effect of the control light.
Furthermore, with OASLM 5, the readout beam has to pass completely through the device. This means that OASLM 5 cannot be opaque. Unfortunately, however, current monolithic fabrication techniques generally require that semiconductor layers be grown on a substrate which is effectively opaque. As a result, OASLM 5 is not capable of being fabricated using a monolithic process.